This invention relates to a center stub still railway tank car, and more specifically to the construction car ends, and even more particularly to the construction of the center stub sill and the cradle assembly which is utilized to join the tank of a railway tank car to the center stub sill assembly.
Center stub sill railway tank cars, for carrying a variety of liquid, gaseous, and semi-liquid ladings, are known in which a horizontal, generally cylindrical tank is utilized not only as a tank to hold the lading, but also as the primary structure for the railway car to carry longitudinal train loads and lading loads. Conventionally, as shown in FIG. 2 herein and as will be hereinafter described in detail, prior art center stub sill railway tank cars had an elongate stub sill assembly secured to each bottom end of the tank, with the stub sill assembly carrying a coupler at its outer end and having a truck assembly pivotally connected thereto for rollingly supporting one end of the tank car on the rails. Typically, a cradle assembly was provided at each end of the car with the cradle assembly comprising a circumferential cradle extending arcuately up from the bottom center of the car, and a cradle pad extending inboard (i.e., toward the center of the car) from the circumferential cradle. Both the cradle and the cradle pad were of steel plate construction and were formed to conform to and to fit on the outer surface of the cylindric tank. Both the cradle and the cradle pad were welded all around their respective peripheries to the outside of the tank, except in certain areas which were inaccessible for welding.
The center stub sill was generally formed of two hot rolled steel Z-shaped sections seam welded along their upper longitudinal centerline so that in cross section, the center stub sill appeared to be a downwardly facing channel-shaped member having lower, outwardly extending feet, a pair of spaced vertical side flanges, and a horizontal top sill member. The center stub sill assembly carried a downwardly facing center plate which permitted the truck assembly to be pivotally attached to the stub sill assembly. The center stub sill and the end of the car were reinforced by a transversely extending bolster assembly welded to the stub sill and to the circumferential pad. Inboard of the outermost edge of the cradle pad, the top web of the center stub sill assembly was cut away such that only the spaced vertical flanges of the stub sill extended inwardly from the outermost edge of the bolster pad. The upper edges of these sill flanges contacted the bottom face of the cradle pad, and the sill flanges were welded to the cradle pad.
Because the center stub sill assembly and the coupler carried thereby were located somewhat below the level of the bottom of the tank (about 8-12 inches below the bottom of the tank), and because the cylindrical tank structure carried the longitudinal train loads axially of the car, an offset or moment arm between the tank structure and the centerline of the coupler was present. This offset resulted in a significant overturning moment being induced in the center stub sill and in the end portion of the tank such that the end of the tank and the center stub sill assembly must withstand these overturning moments. It will be appreciated that the longitudinal train loads that the car is required to withstand, in accordance with the American Association of Railroads (AAR), is a dynamic or impact load of 1,250,000 pounds and a static squeeze of compression load of about 1,000,000 pounds. Because of the vertical offset and the magnitude of the loads, the overturning moments are very significant.
Additionally, the above-noted overturning moments force the inboard end of the center stub sill assembly to apply relatively high, localized loading to the bottom of the tank such that even though the bottom of the tank was structurally reinforced by the plate-like cradle pad proximate the inboard ends of the center stub sill, an additional cradle pad reinforcement plate was required so as to prevent localized buckling of the cradle pad and the tank.
Further, it is conventional to provide an exterior heater coil system on many tank cars in which half-oval steam lines are welded to the exterior surface of the tank in an elongate, serpentine fashion so as to extend lengthwise of the tank substantially from one end thereof to the other, and to extend up the sides of the tank to about mid-level of the tank. For unloading certain ladings such as heavy petroleum oils, molten sulphur, tallow, lard, and other ladings which may congeal during shipping and which may need to be heated for ease in unloading via a bottom outlet valve, a train attendant, upon unloading of the car, would connect a steam line to the exterior heater coil system provided on the car such that live steam could be circulated through the steam lines thereby to heat the lading within the tank car.
In the design of such a heater coil system, it is desirable that the bottom centerline of the car be heated so as to ensure that all of the lading within the car may be readily off-loaded by means of the bottom outlet valve. It is also important in most exterior heater coil systems that the end of the serpentine heater coils extend outboard as far as possible so as to heat the lading at the ends of the tank car.
In prior art tank car constructions, as heretofore described, it oftentimes was necessary, as shown in FIG. 2 herein, for the serpentine steam coils to extend through the bolster web and to overlie the circumferential cradle so as to heat the ends of the tank. In order to overlie the circumferential cradle, it was necessary to provide a joggle in the steam coils which was difficult and expensive to fabricate, and which required additional labor. Likewise, the provision of cutouts in the bolster web required additional fabrication and labor, and it was also necessary to ensure that the cutouts for the steam coils in the bolster web were structural cutouts such that the load carrying capability of the bolster web was not adversely affected.
Still further, with a longitudinal cradle pad conforming to and welded to the bottom center of the tank, it was difficult to effectively heat the bottom portion of the tank covered by the cradle pad. If a heater coil were mounted on the bottom centerline of the car, it must overlie the longitudinal cradle pad and be separated from the interior of the tank by both the thickness of the tank wall and by the thickness of the cradle pad. This oftentimes could mean as much as three-fourths inch of steel or more with an effective thermal conduction barrier at the interface between the longitudinal cradle pad and the tank substantially reduced heat transfer from this steam coil in heat transfer relation with the cradle pad. Additionally, the cradle reinforcement pad proximate the ends of the center stub sill, as heretofore described, even further interfered with a bottom centerline steam coil.
Still further, in the construction of prior art center stub sill railway tank cars utilizing hot rolled Z-section center stub sills, it was a relatively difficult and labor intensive task to weld the center stub sill weldment to the cradle assembly. Since the center stub sill member was typically made by butt seam welding the upper flanges of two Z-sections together along the entire length of the stub sill during fabrication thereof, it was necessary to cut away the upper web of the center sill in the area inboard of the outermost edge of the cradle so that the stub sill vertical flanges could be welded to the cradle pad inboard of the cradle pad. The necessity of cutting away the top web of the center sill required extensive labor and resulted in significant material waste. Additionally, because the center stub sill, bolster assembly, and cradle assembly are welded together as a single weldment, it was a relatively difficult task to fit the cradle pad to the cylindric tank.
Further, in the securement of the bolster assembly to the center stub sill and to the cradle, there were a number of structural steel plates installed within the center sill, such as center filler spacers and the like, which were difficult to fit and to weld. There were also areas between the backstop brace and the cradle pad which did not have sufficient access such that the cradle could be readily welded to the tank, and thus it was necessary to omit the weld from these areas. To compensate for this inability to weld, it was oftentimes necessary to provide additional structure or slot welds which increased weight and cost of the stub sill assembly. Still further, outboard of the bolster assembly, on opposite sides of the center stub sill, it was oftentimes necessary to provide sill reinforcing bars on opposite sides of the sill which extended longitudinally of the sill for a distance of approximately three feet or so (1 m.). These sill reinforcing bars provided additional tension and compression load carrying capability outboard of the bolster assembly for the center stub sill, but increased the cost and weight of the assembly.
With prior art cradle assemblies utilizing a cradle pad or plate conformed in transverse direction to the cylindric shape of the tank welded around its periphery to the tank bottom, it will be appreciated that it was a difficult task to conform the cradle pad to the cylindric surface of the tank. This is due to the fact that the pad has a length of approximately 12-13 feet and a width of about 3 feet, and the cradle, center stub sill and bolster are welded into a rigid weldment before it is welded to the tank.
It was also recognized that weld terminations of the prior art railway car, such as is illustrated in FIG. 2, in the region of the bolster, posed a load transfer problem in the center stub sill construction where longitudinal and vertical loads were reacted at their respective weld terminations. It is desirable to prevent the longitudinal and vertical loads from being reacted at their weld terminations. However, with the prior art center stub sill and cradle pad design, it was, in many cases, nevertheless necessary to react these loads at their weld terminations.